The present invention relates, in general to wireless signal transmission systems and, more particularly, to stereo sound wireless transmission systems.
In wireless transmission systems particularly suited for transmitting a radio and/or an audio portion of a television broadcast, and, even more particularly, to wireless transmission systems for music, stereo sound is desirable.
While two separate signals are combined in stereophonic transmission systems, the sound signals are spatially divided into two 50 Hz to 15 kHz audio left and right channels. Music or sounds that originate on the left side are reproduced only in the left speaker, with music or sounds that originated on the right side reproduce only on the right speaker.
While two separate transmitter and receiver systems could be used for separately broadcasting the left channel and right channel sounds to stereo speakers, headphones, etc., the present state of the art utilizes frequency division multiplexing to form a composite baseband signal as shown in FIG. 1. A standard FM stereo system uses frequency division multiplexing to combine the left and right channel signals which are in the 50H3 passband. The left and right channel signals are added to produce a sum signal and, also, subtracted from one another to produce a difference signal. The sum signal is a monophonic signal which is used for broadcast from a single speaker. The different signal is used as a double-sideband suppressed carrier to modulate a 38 kHz sine wave.
The double side band suppressed carrier signal is added to the sum signal and the combination sent on the transmitter's FM modulator. Other transmission frequencies in the 900 mHz range or in the 2 GHz frequency band may also be used to form the carrier on which the left and right signal information is modulated.
A monophonic receiver can filter the signals to block signals above 15 kHz and thereby reproduce only monophonic sum signal. A stereo receiver has an additional circuit after the FM demodulator which can detect a 19 kHz pilot tone which double the 38 kHz carrier signal.
Once the stereo receiver has detected the 19 kHz pilot tone indicating a stereo transmission, the stereo receiver recovers the difference information by demodulating the received signal to create the sum plus the difference signally and the sum minus the difference signals, to recreate the left and right signals for broadcast by the left and right speakers.
While the FM broadcast system effectively transmits and relatively accurately reproduces stereo sounds, FM signals are prone to interference by fading by reflection off of walls, particularly in the case of mobile receivers, such as stereo headphones as the individual wearing the headphones walks around a building and, also, by channel bleed over.
Thus, it would be desirable to provide an FM transmission and receiver system which has increased a signal to noise ratio to prevent bleed over and interference and thereby be able to recreate stereo signals having a signal to noise ratio closer to the 90 db signal to noise ratio of CDs. It would also be desirable to provide a means for compensating for fade out of one of the stereo channels, particularly in a portable receiver, as a particular channel signal decreases in signal to noise ratio.